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Selecting the Right Ultra-Low Quiescent-Current LDO Regulator

By Ashok Bindra

Contributed By Electronic Products


The use of low-dropout regulators, popularly known as LDOs, is common in many applications today because they provide a simple and inexpensive way to regulate an output voltage that is stepped-down from a higher input voltage. In addition, linear LDO voltage regulators contribute very-low noise as compared to switching regulators.

Nonetheless, to keep system power consumption low, such regulators must also feature ultra-low quiescent current (IQ) while providing excellent dynamic performance to ensure a stable, noise-free voltage rail, suitable for driving IC loads such as microprocessors, FPGAs, and other devices on the system board.

In reality, ultra-low IQ and good dynamic response characteristics do not go hand-in-hand. In fact, two similar LDOs with identical IQ current specs can differ considerably in terms of dynamic performance. According to an ON Semiconductor application note,¹ these two requirements are often mutually exclusive and pose a real challenge to power-IC designers. Consequently, not many LDOs available on the market can satisfy both of these requirements simultaneously.

Biasing ultra-low-IQ LDOs

According to ON Semi, the two main factors influencing the dynamic performance of an ultra-low-IQ LDO regulator are the process technology used to fabricate the device and the associated circuit design. While an advanced process such as CMOS or BiCMOS can be optimized for low power consumption and high-speed performance of the power device, the dynamic performance is dependent on the circuit design. Combining these two techniques, ON Semiconductor’s power-IC designers have accomplished a lot more. Besides delivering ultra-low IQ with excellent line and load transients, available LDOs also feature ultra-low output noise and high power supply rejection ratio (PSRR) characteristics.

Similar advances have also been made by other suppliers including Linear Technology Corp., Maxim Integrated Products, and Texas Instruments, to name just a few. To address a variety of battery-powered mobile applications, these LDO suppliers have crafted ultra-low-IQ LDOs with high PSRR, ultra-low noise and fast transient response characteristics.

Traditionally, ultra-low-IQ CMOS LDOs use a constant biasing scheme to keep ground current (IGND) consumption relatively constant across the available range of output currents. By definition, IQ defines IGND. ON Semiconductor’s MC78LC is a good example of such a device, featuring an IGND (or IQ) of 1.5 µA. The primary disadvantage of constant biasing is relatively poor dynamic performance, namely load and line transients, PSRR, and output noise, as ON Semi engineers report in the application note. ON Semi suggests tweaking this performance using larger output capacitors. Figure 1 shows LDO MC78LC’s load-transient overshoot and undershoot are improved by increasing the output capacitor (COUT) from 1 to 100 µF.

ON Semiconductor MC78LC’s load-transient overshoot and undershoot

Figure 1: MC78LC’s load-transient overshoot and undershoot are significantly improved by increasing the output capacitor (COUT) from 1 to 100 µF.

Table 1 depicts precisely the overshoot and undershoot amplitudes for three different output capacitor values. It can be seen that the transient amplitude was greatly reduced by using a larger 100 µF output capacitor.

  Output Capacitor, COUT
1 μF 10 μF 100 μF
Overshoot +560 mV +180 mV +80 mV
Undershoot -720 mV -240 mV -100 mV

Table 1: MC78LC’s transient amplitudes for three different output capacitor COUT values.

While the LDO’s transient amplitude decreases sharply with a larger output capacitor, the settling time increases concurrently. Furthermore, the application note suggests that when using large output capacitors it may be necessary to provide an external reverse-protection diode between the VIN and VOUT pins. It protects the LDO regulator from the excessive reverse current that could otherwise flow through the internal PMOS body diode during a sudden fall of the input voltage. However, ON Semiconductor’s product marketing engineer Pawel Holeksa cautions that increasing COUT does not guarantee the desired performance. Additionally, larger output capacitors and external diodes for protection add to the cost and size of the solution.

Low noise, high PSRR

Consequently, to overcome the limitations of constant biasing, ON Semiconductor has developed LDOs using a clever biasing scheme. By changing the ground current or IQ current proportionally to the output current, the new LDOs improve the relatively-poor dynamic behavior of the constant-bias LDOs. Two such examples are the NCP4681 and NCP4624 featuring typical quiescent current of 1 µA and 2 µA, respectively. Figure 2 demonstrates the concept used in these ultra-low-IQ LDOs wherein IGND current rises proportionally to the output current. Also, it is observed that the IGND starts to rise at IOU > 2 mA.

ON Semiconductor NCP4681 and NCP4624

Figure 2: In these ultra-low-IQ LDOs, NCP4681 and NCP4624, the ground-current IGND rises proportionally to the output current.

In comparison to constant IGND LDOs, the data sheets for NCP4681/NCP4624 regulators show that proportional-bias LDOs offer significant improvement in power supply rejection ratio (PSRR) and load transient performance. By comparison, NCP4681 shows about 15 dB improvement in PSRR specification at 100 Hz and IOUT = 30 mA. At this current and frequency ratings with 1.5 V output and 2.5 V input, the product datasheet shows 53 dB for the PSRR rating, which remains the same as the output current drops to 1 mA. Although, the proportional-bias technique provides improved dynamic parameters with respect to constant-IGND LDOs, it may not be sufficient for some demanding applications.

In fact, some applications demand even better PSRR performance from ultra-low-IQ LDOs. Hence, LDO noise and PSRR are other important specifications that must be taken into account, in addition to transient response, when selecting ultra-low-IQ LDO regulators. To address such needs, Texas Instruments has developed the TPS727xx family of ultra-low-IQ LDOs with very-high PSRR and ultra-low noise and excellent transient response. TI attributes this performance to using advanced BiCMOS process and a PMOS FET pass device. The 250 mA member TPS72718 for instance, specs IQ of 7.9 µA with 70 dB PSRR at 1 kHz when the output voltage is 1.8 V and input is 2.3 V at 10 mA output current (Figure 3). For similar input and output conditions with a bandwidth of 100 Hz to 100 kHz, the output noise voltage is only 33.5 µVrms.

Texas Instruments TPS72718 LDO

Figure 3: The ultra-low-IQ LDO TPS72718 exhibits a PSRR of >70 dB at 1 kHz when the input-to-output differential is 0.5 V and output current is 10 mA.

Likewise, to enhance dynamic parameters and still maintain ultra-low IQ, ON Semiconductor has implemented a new technique called adaptive ground current. Using this technique, LDOs can boost the ground current at a certain level of output current without compromising dynamic performance. As a result, the load/line transients and PSRR performance is excellent with minimal output noise. Such devices are being optimized for powering sensitive analog/RF circuitry in environments requiring long battery life and small-solution footprint.

In summary, modern LDOs are combining the benefits of process technologies and circuit design to deliver ultra-low-IQ LDOs without compromising high performance levels of dynamic parameters such as load transients, PSRR, and output noise.

For more information on the products discussed in this article, use the links provided to access product pages on the Digi-Key website.

Reference
  1. Application Note AND9089/D, “A Guide to Choosing the Right Ultra-Low IQ Low Dropout Linear Voltage Regulators” , ON Semiconductor.

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Ashok Bindra

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Electronic Products

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